The process of making semiconductors, e.g., integrated-circuit transistors, involves numerous processes carried out under very low pressures. These pressures are maintained in what are commonly referred to as “vacuum chambers.” In general, a vacuum chamber is an enclosure connected to a pumping system, e.g., one including a cryo pump or turbo pump. The pumping system maintains low or extremely low pressures, e.g., 10−8 Torr for a base pressure, and certain pressure, e.g., 5 mTorr during processing. The pumping system can optionally maintain specified concentrations of selected gasses in the chamber. An example of a cluster tool using chambers is the ENDURA physical vapor deposition (PVD) machine made by APPLIED MATERIALS. For example, PVD processes for depositing Cu and Ta(N) require high vacuum, e.g., ˜5 mTorr. Throughout this disclosure, “vacuum” refers to pressures much lower than atmospheric (1 atm=760 Torr), e.g., <20 Torr.
The health of the vacuum system can be monitored in a variety of ways. For example, the pressure in the chamber can be plotted over time. Pressure increase can result from outgassing from moisture or other materials in the chamber, e.g., materials such as hydrocarbons coating the surface of the chamber or process kits. Pressure increase can also result from leaks between the chamber and the outside atmosphere, or between the chamber and its pumping or other components. For example, a leak in a cutoff valve can leak process gases, e.g., N2 or Ar, into the chamber.
Since a 300 mm wafer can cost thousands of dollars, early detection of failures, i.e., leaks, can greatly improve the economic viability of a fab. Various in-situ chamber leak detection methods have been developed. For example, residual gas analyzers (RGAs) have been used to test chambers. RGAs perform mass spectroscopy on molecules in chambers to determine the composition of those molecules or their partial pressures. However, RGA equipment is bulky and expensive and the operating lifetime of the equipment is too low to apply it to every process chamber. Alternatively, oxygen (O2) sensors have been used with some process chambers, such as Rapid Thermal Processing (RTP) chambers, for leak detection. However, the sensitivity of oxygen sensors is too poor for process chambers operating at low pressures.
Plasma assisted optical emission spectroscopy (SPOES) is suitable due to its low cost, small size, and long term stability. However, plasma OES is subject to a variety of disadvantages, such as a narrow operating pressure range. For example, some SPOES sensors have an operating pressure in the range of 10 mtorr to 1 torr, which pressure range is too low to maintain the plasma. Additionally, the sensitivity of the SPOES sensor varies for various gases. For example, the SPOES sensor is more sensitive for detecting nitrogen (N2) compared to oxygen (O2). In some SPOES sensors, the detection limit for nitrogen in 100 mTorr argon (Ar) is about 1 parts-per-million (ppm) while the detection limit for oxygen is greater than 100 ppm. Further, SPOES sensors are subject to interference from background gases, residual process gases, by-product gases, and other chambers.
As used herein, “measuring a chamber” can include measuring the pressure in a chamber, partial pressures of various gasses, or composition of the atmosphere in a chamber; or testing for or detecting leaks.